docs: def add_post_processing(effect,  **options):
      """Apply a post processing effect.
  
      **Parameters**
      
      effect: string
          The effect to be applied, choose between ``ssao``,
          ``outline``, ``fxaa``, ``gamma``.
      
      **options:
          Options used to initialize the effect, check the
          :doc:`chemlab.graphics.postprocessing` for a complete
          reference of all the options.
      
      **Returns**
  
      A string identifier that can be used to reference the applied effect.
  
      """
  
      from chemlab.graphics.postprocessing import SSAOEffect, OutlineEffect, FXAAEffect, GammaCorrectionEffect
      
      pp_map = {'ssao': SSAOEffect,
                'outline': OutlineEffect,
                'fxaa': FXAAEffect,
                'gamma': GammaCorrectionEffect}
      
      pp = viewer.add_post_processing(pp_map[effect], **options)    
      viewer.update()
  
  
      global _counter
      _counter += 1
      
      str_id = effect + str(_counter)    
      _effect_map[str_id] = pp # saving it for removal for later...
      
      return str_id